This invention relates to a modified natural clinoptilolite for use as a catalyst in a process for the conversion of methanol and ammonia to methyl amines, and to a method for the conversion of methanol and ammonia using the catalyst.
Methyl amines, particularly monomethylamine and dimethylamine, are important chemical intermediates for use as starting materials for the manufacture of various solvents, pharmaceuticals, organic rubbers and surfactants. There is therefore a need to identify improved manufacturing processes for these chemicals. Methyl amines are generally produced by reacting methanol with ammonia in the gas phase at an elevated temperature (250.degree. C.-400.degree. C.) in the presence of a suitable catalyst. For example, a mixture of monomethylamine, dimethylamine and trimethylamine can be produced by reacting ammonia and methanol over a mixture of zinc chloride and ammonium chloride as catalyst at 300.degree. C. (see J D Roberts and M C Caserio in "Basic Principles of Organic Chemistry", p. 654, W A Benjamin, New York 1965).
In general, acidic catalysts give the best conversions. The main problem with this process is that the reaction produces an equilibrium mixture of the different methyl amines with the trimethylamine being the most thermodynamically favoured product. This is unfortunate since commercially the trimethylamine is the least desired product. More recently the use of crystalline aluminosilicates (known as zeolites), as catalysts has been found to produce mixtures of amines rich in the more commercially valuable dimethylamines. For example, L Abrams, T E Gier, P D Shannon and G C Sonnischen have disclosed (in European Patent Application No. 183423) that methanol and ammonia can be reacted at 250.degree. C.-450.degree. C. and 1 atmosphere pressure over the zeolites rho, ZK-5 or chabazite (sometimes exchanged with H.sup.+ or alkali metal ions) to give dimethylamine as the major product. R N Cochran has also disclosed (in European Patent Application No. 85408 and U.S. Pat. No. 4,398,041) that crystalline aluminosilicates catalyse the reaction of methanol with ammonia to form methyl amines. The preferred crystalline aluminosilicate zeolites are the hydrogen form of erionite or macroporous chabazite/erionite mixtures in the hydrogen form. K M Minnachev, A I Maksimov, I V Mishin and I I Leviskii have demonstrated (in the Russian Journal Dokl. Acad. Nauk. SSSR, 1985, volume 280, part 5, pages 1154-9) that butanol can be reacted with ammonia using a range of zeolites as catalysts at 400.degree. C. to form mainly monobutylamine and dibutylamine together with traces of tributylamine. The faujasite type zeolite Y in the sodium form was found to give the best results whereas the sodium form of zeolite A gave the poorest results. In addition a number of studies have shown that the pentasil zeolites, e.g. zeolite ZSM-5, can also be used as catalysts for the synthesis of methyl amines.
Other relevant references include U.S. Pat. No. 4,806,689 which discloses the production of dimethylamine by reacting methanol or dimethylether and ammonia in the presence of a catalytic amount of acidic zeolite rho;
U.S. Pat. No. 4,752,596 which discloses the production of dimethylamine by reacting methanol or dimethylether and ammonia in the presence of a catalyst which is an acidic zeolite selected from chabazite, eriomite, ZK-5 and rho optionally modified by treatment with compounds containing silicon, aluminium, phosphorous or boron; PA1 Japanese Patent Application No. 90027335 which discloses the preparation of methylamine by reacting methanol and ammonia in the presence of a mordenite type zeolite; and PA1 an article in J Catal. 1990, vol 124,.No. 1, pages 268 to 280 which discloses designing zeolite catalysts for shape-selective reactions, and chemical modification of surfaces for improved selectivity to dimethylamine in synthesis from methanol and ammonia. PA1 (a) feeding the methanol and the ammonia to the reactor containing the catalyst; PA1 (b) converting the methanol and the ammonia in the reactor in the presence of the catalyst at a temperature of from 300.degree. C. to 500.degree. C. inclusive, preferably from 350.degree. C. to 450.degree. C. inclusive, and at a pressure from 1 to 20 atmospheres inclusive, preferably from 1 to 10 atmospheres inclusive, more preferably from 1 to 3 atmospheres inclusive; and PA1 (c) recovering the product.
Under conditions normally employed with commercial catalysts, the aforementioned zeolites tend to produce mixtures of alkyl amines. The chemical industry would prefer a catalyst that has high specificity to a desired product, e.g. monomethylamine, since this would reduce product separation and manufacturing costs. Furthermore, the chemical industry would prefer a catalyst that is relatively inexpensive and this is not readily achieved if synthetic zeolites are utilised.